Structured Data and Classes

Chapter 7

Combining Data into Structures

Structure: C++ construct that allows multiple variables to be grouped together

Structure Declaration Format: struct structure name{type1 field1;type2 field2; …

typen fieldn;};

Example struct Declaration

struct Student

{

int studentID;

string name;

short year;

float gpa;

};

structure tag

structure members

Notice the required

;

struct Declaration Notes

struct names commonly begin with an uppercase letter

Multiple fields of same type can be in a comma-separated list string name,

address;

Defining Structure Variables struct declaration does not

allocate memory or create variables

To define variables, use structure tag as type nameStudent s1;

studentID

name

year

gpa

s1

Accessing Structure Members

Use the dot (.) operator to refer to members of struct variables

getline(cin, s1.name); cin >> s1.studentID;

s1.gpa = 3.75; Member variables can be used in

any manner appropriate for their data type

Displaying struct Members

To display the contents of a struct variable, you must display each field separately, using the dot operator

Wrong: cout << s1; // won’t work!Correct: cout << s1.studentID << endl; cout << s1.name << endl; cout << s1.year << endl; cout << s1.gpa;

Initializing a Structure Cannot initialize members in the

structure declaration, because no memory has been allocated yet

struct Student // Illegal { // initialization int studentID = 1145; string name = “Alex”; short year = 1; float gpa = 2.95; };

Initializing a Structure

Structure members are initialized at the time a structure variable is created

Can initialize a structure variable’s members with either an initialization list a constructor

Using an Initialization List

An initialization list is an ordered set of values, separated by commas and contained in { }, that provides initial values for a set of data members

{12, 6, 3} // initialization list // with 3 values

More on Initialization Lists

Order of list elements matters: First value initializes first data member, second value initializes second data member, etc.

Elements of an initialization list can be constants, variables, or expressions

{12, W, L/W + 1} // initialization list // with 3 items

Initialization List Example

Structure Declaration Structure Variable

struct Dimensions{ int length, width, height;};

Dimensions box = {12,6,3};

box

length 12

width 6

height 3

Partial Initialization

Can initialize just some members, but cannot skip over members

Dimensions box1 = {12,6}; //okayDimensions box2 = {12,,3}; //illegal

Problems with Initialization List

Can’t omit a value for a member without omitting values for all following members

Does not work on most modern compilers if the structure contains any string objects Will, however, work with C-string members

Using a Constructor to Initialize Structure Members

A constructor is a special function that can be a member of a structure

It is normally written inside the struct declaration

Its purpose is to initialize the structure’s data members

Using a Constructor (continued)

Unlike most functions, a constructor is not called; instead, it is automatically invoked when a structure variable is created

The constructor name must be the same as the structure name (i.e. the struct tag)

The constructor must have no return type

A Structure with a Constructor

struct Dimensions{ int length, width, height; // Constructor Dimensions(int L, int W, int H) {length = L; width = W; height = H;}};

Passing Arguments to a Constructor

Create a structure variable and follow its name with an argument list

Example:

Dimensions box3(12, 6, 3);

Default Arguments

A constructor may be written to have default arguments

struct Dimensions{ int length, width, height; // Constructor Dimensions(int L=1, int W=1, int H=1) {length = L; width = W; height = H;}};

Examples

//Create a box with all dimensions given Dimensions box4(12, 6, 3);

//Create a box using default value 1 for //height Dimensions box5(12, 6);

//Create a box using all default values Dimensions box6;

Omit () when no arguments

are used

Nested Structures

A structure can have another structure as a member.

struct PersonInfo { string name, address, city; }; struct Student { int studentID; PersonInfo pData; short year; float gpa; };

Members of Nested Structures

Use the dot operator multiple times to dereference fields of nested structures

Student s5;

s5.pData.name = "Joanne";

s5.pData.city = "Tulsa";

Structures as Function Arguments

May pass members of struct variables to functions computeGPA(s1.gpa);

May pass entire struct variables to functions showData(s5);

Can use reference parameter if function needs to modify contents of structure variable

Notes on Passing Structures

Using a value parameter for structure can slow down a program and waste space

Using a reference parameter speeds up program, but allows the function to modify data in the structure

To save space and time, while protecting structure data that should not be changed, use a const reference parameter

void showData(const Student &s) // header

Returning a Structure from a Function

Function can return a structStudent getStuData(); // prototype

s1 = getStuData(); // call Function must define a local

structure for internal use to use with return statement

Returning a Structure Example

Student getStuData() { Student s; // local variable cin >> s.studentID; cin.ignore(); getline(cin, s.pData.name); getline(cin, s.pData.address); getline(cin, s.pData.city); cin >> s.year; cin >> s.gpa; return s; }

Unions Similar to a struct, but

all members share a single memory location (which saves space) only 1 member of the union can be used at a time

Declared using key word union Otherwise the same as struct Variables defined and accessed like struct variables

Example union Declaration

union WageInfo{

float hourlyRate; float annualSalary;};

union tag

Notice the required

;

union members

Anonymous Union

A union without a tag:union { ... };

With no tag you cannot create additional union variables of this type later

Allocates memory at declaration time Refer to members directly without

dot operator

Anonymous Union

Exampleunion

{

int weightLbs;

float weightOz;

};

Abstract Data Types

A programmer-created data type that specifies legal values that can be stored operations that can be done on the values

The user of an abstract data type does not need to know any implementation details (e.g., how the data is stored or how the operations on it are carried out)

Abstraction and Data Types Abstraction: a definition that captures

general characteristics without details Ex: abstract triangle is a 3-sided polygon.

A specific triangle may be scalene, isosceles, or equilateral

Data Type: defines the kind of values that can be stored in a variable and the operations that can be performed on it

Object-Oriented Programming

Procedural programming focuses on the process/actions that occur in a program

Object-Oriented programming is based on the data and the functions that operate on it. Objects are instances of ADTs.

Limitations of Procedural Programming

Use of global data may allow data corruption

Programs are often based on complex function hierarchies difficult to understand and maintain difficult to modify and extend easy to break

Object-Oriented ProgrammingTerminology

class: similar to a struct

Allows bundling of related variables (member data) and the functions that operate on them (member functions)

Describes the properties that all instances of the class will have

object: an instance of a class, in the same way that a variable can be an instance of a struct

More Object-Oriented Programming Terminology

attributes: member data of a class methods or behaviors: member

functions of a class data hiding: restricting access to

certain members of an object

Public Interface Class objects are accessed from outside

the class via a public interface This is done by allowing some of the member

functions to be called from outside the class Normally all the class’s member variables are

accessed only through these “public” functions provides some protection from data corruption

Introduction to Classes A class declaration describes the

member variables and member functions that its objects will have

It is a pattern for creating objects Class Declaration Format: class className { declaration; declaration; }; Notice the

required ;

Access Specifiers Used to control access to members of the

class. Each member is declared to be eitherpublic: can be accessed by functions outside of the classorprivate: can only be called by or accessed by functions that are members

of the class

Class Example

class Square {

private: int side;

public: void setSide(int

s) { side = s; } int getSide() { return side; }

};

Access specifiers

More on Access Specifiers

Can be listed in any order in a class

Can appear multiple times in a class

If not specified, the default is private

Introduction to Objects An object is an instance of a class Defined like structure variables

Square sq1, sq2; Access members using dot operator

sq1.setSide(5);cout << sq2.getSide();

Compiler error if attempt to access private member using dot operator

Defining Member Functions

Member functions are part of a class declaration

Can place entire function definition inside the class declaration

or Can place just the prototype inside

the class declaration and write the function definition after the class

Defining Member Functions Inside the Class Declaration

Member functions defined inside the class declaration are called inline functions

Only very short functions, like the one below, can be inline functions

int getSide() { return side; }

Inline Member Function Example

class Square { private: int side; public: void setSide(int s) { side = s; }

int getSide() { return side; }

};

inline functions

Defining Member Functions After the Class Declaration

To define a member function after the class declaration Put a function prototype in the class declaration In the function definition, precede function

name with class name and scope resolution operator (::)

int Square::getSide(){ return side;}

Tradeoffs of Inline vs. Regular Member Functions

When a regular function is called, control passes to the called function the compiler stores return address of

call, allocates memory for local variables, etc.

Code for an inline function is copied into the program in place of the call larger executable program but no function call overhead, hence

faster execution

Design Considerations

Class should be designed to provide functions to store and retrieve data

In general, I/O should be done by functions that use class objects, rather than by class member functions (Exceptions can occur, as with a class designed to display a menu)

More Design Considerations

Separating class declaration, member function definitions, and program that uses the class into separate files is considered good design

Using Separate Files

Place class declaration in a header file that serves as the class specification file. Name the file classname.h (for example, Square.h)

Place member function definitions in a class implementation file. Name the file classname.cpp (for example, Square.cpp)This file should #include the class specification file.

Programs that use the class must #include the class specification file and be compiled and linked with the class implementation file.

Using a Constructor with a Class

As with a structure, a constructor can be used to initialize data members of a class

Must be a public member function Must be named the same as the class Must have no return type Is called automatically when an object

of the class is created

The Default Constructor

Constructors can have any number of parameters, including none

A default constructor is one that takes no arguments either due to No parameters or All parameters have default values

Default Constructor Example

class Square{ private: int side;

public: Square() // default { side = 1; } // constructor

// Other member // functions go here };

Has no parameters

Another Default Constructor Example

class Square{ private: int side;

public: Square(int s = 1) // default { side = s; } // constructor

// Other member // functions go here };

Has parameter but it has a default value

Invoking a Constructor Invoking a constructor for a class object is

done just like invoking a constructor for a structure variable

To create an object using the default constructor, use no argument list and no ()Square square1;

To create an object using a constructor that has parameters, include an argument list

Square square1(8);

Overloading Constructors

A class can have > 1 constructor Overloaded constructors in a class

must have different parameter lists

Square(); Square(int);

Only one default constructor is allowed

Member Function Overloading

Non-constructor member functions can also be overloaded void setSide();

void setSide(int); Must have unique parameter lists

(as with constructors)

Destructors Public member function automatically

called when an object is destroyed Destructor name is ~className, e.g.,

~Square Has no return type Takes no arguments Only 1 destructor is allowed per class (i.e., it cannot be overloaded)

Input Validation Objects

Objects can be designed to validate user input

to ensure acceptable menu choice to ensure a value is in range of valid

values etc.

Using Private Member Functions

A private member function can only be called by another member function of the same class

It is used for internal processing by the class, not for use outside of the class